The largest source of industrial waste-water which is discharged to the environment is blowdown from cooling systems. Over 30 billion gallons per day are discharged in the United States from industrial facilities, and this figure does not include utilities which are the largest users of cooling water. Blowdown can contain heat, toxic chemicals--in the form of water conditioning agents, such as, corrosion and scale inhibitors, and disperants--and also have a high degree of dissolved solids. Thus, blowdown which is discharged to the environment constitutes a major source of pollution.
By reason of the high concentration of dissolved solids in raw cooling water, including hardness such as calcium and magnesium compounds, which makes it scaling and corrosive to metallic parts of cooling systems such as heat exchangers, it is customary to add corrosion inhibitors such as soluble alkali metal chromates to the cooling water as well as softening agents and scale inhibitors such as phosphonates, polymaleic anhydrides or polyacrylates. Dissolved silica is a major scale forming material found in cooling water. It is the most ubiquitous material in cooling water and no known inhibitor, chelant or dispersant exists which will significantly control its tendency to form scale when its concentration in a cooling water system exceeds its solubility limit of roughly about 150 to about 200 milligrams per liter. Above such limits it polymerizes to form scale on heat exchange surfaces. It may also react with multivalent cations, such as magnesium and calcium, to form scale.
Heretofore, the only practical method employed to control the silica concentration in cooling water systems has been to discharge a blowdown stream from such systems to the environment when, due to water evaporation from the system, the silica concentration begins to approach its limits of solubility. Thereafter, fresh water makeup of low silica concentration is added to the system to compensate for the water deficiency created by the blowdown and evaporation from the system, and the total silica concentration of the system is reduced to a safe level.
The blowdown method of silica control is undesirable in at least two important respects. First, the fresh water makeup to replace the blowdown water losses must be treated with chemical additives for water softening, and corrosion and scale inhibition to condition it for use in a cooling water system--at a cost which, on an industry wide level, has been estimated at about five billion dollars a year. The value of such treating chemicals which are lost from the system in blowdown have been estimated at a billion dollars per year. This represents an expense which could be avoided if blowdown could be conditioned for reuse without removal of such chemical additives. Secondly, and more importantly, the blowdown must eventually be released to the environment and by reason of the chemical conditioning agents it contains thus constitutes it as major source of pollution. In many instances where local regulations exist on chromate discharges to natural streams and sewer systems and the like, it is necessary to treat the chromate containing blowdown by costly ion exchange or chromate distruction processes to reduce its chromate content to acceptable levels prior to discharge. Even then, the non-chromate chemical additives and highly dissolved solids such as alkali metal chlorides, carbonates and sulfates are discharged to the environment as pollutants. Aside from the natural desire which has always existed to eliminate pollution sources such as blowdown there is also an increasing concern evidenced by recent environmental legislation to strictly regulate discharges from cooling water systems with the ultimate goal to totally eliminate the discharge of pollutants contained therein.
Previous efforts to remove silica from the blowdown streams of cooling water systems such as by softening, reverse osmosis, demineralization and/or evaporation processes are non-selective in that not only silica but the other beneficial water conditioning chemicals are eliminated from the treated water as well. Ion exchange methods have also been attempted for silica removal, but have proved generally unacceptable because the high sulfate content of cooling water excludes absorption of silica by conventional ion-exchange resins. Ion exchange resins can only be employed for silica removal from cooling water if such water is first demineralized to remove the interfering sulfates. The above mentioned methods have not proved entirely satisfactory and hence have been used only to a limited extent.
Most attention to silica removal from water to prevent scaling has been directed to the treatment of raw water--to prepare it as boiler feed water for steam power production--wherein such raw water has not been previously conditioned by the addition of chemical additives such as chromates, phosphonates and/or sulfates. In such case wherein the removal of silica is the only concern the prior art evidences that a number of researchers in the late 30's and 40's tested a variety of compounds for removal of soluble silica. The compounds tested include ferric sulfate, magnesium sulfate, magnesium oxide, aluminum hydroxide, sodium aluminate and activated alumina.
Thus, although a number of methods exist for silica removal generally such methods are not suitable for application to the treatment of chemically conditioned water such as is normally found in typical cooling water systems.
Activated alumina is known to have silica removing properties. Behrman and Gustafson "Removal of Silica from Water", Industrial Engineering Chemistry, 1940, pp. 468-72, describe a series of experiments on silica removal from raw water utilizing granular activated alumina and found reduction of silica in water from 50 milligrams per liter to 5 milligrams per liter. Additionally, they found that activated alumina could be regenerated for reuse by washing it with a solution of strong acid followed by a solution of strong alkali.
Although activated alumina has long been known as an absorbent for silica in the treatment of raw water such as boiler feed water and the like, to my knowledge heretofore there has been no attempt to apply it to the removal of silica from a water stream containing sulfates and the various dispersants, corrosion and scale inhibitors, and water softeners normally found in typical cooling water systems--particularly chromates and phosphonates.
Activated alumina is known as an absorbent for organic and inorganic compounds in their nonionic, cationic and anionic forms. Activated alumina has sites available for both cation and anion absorption and both cation and anion exchange can occur simultaneously on the alumina surface, although one or the other usually predominates. Activated alumina's characteristic as being an ion absorbent would make it appear to be unsuitable for use in the treatment of cooling water blowdown for silica removal since one would expect the alumina to remove beneficial water treatment additives contained therein and/or that such additives would so saturate the active absorption sites on the alumina surface so as to render it incapable of removing silica.